Disclosed herein is a battery pack including a battery module including a cell module stack having a structure in which a plurality of cell modules, each of which includes a battery cell mounted in a cartridge, is vertically stacked, a lower end plate to support a lower end of the cell module stack, an upper end plate to fix an uppermost cartridge of the cell module stack disposed on the lower end plate, and a voltage detection assembly to detect voltages of the battery cells, a box type pack case in which the battery module is mounted, a pack cover coupled to the pack case, and fastening extension members protruding upward from the battery module to couple the battery module to the pack case and the pack cover.